Defrost control



2 Sheets-Sheet 1 Filed July 23, 1962 EVHP. TEMP.

Jan. 12, 1965 E. F. HUBACKER 3 DEFROST CONTROL Filed July 25, 1962 2 Sheets-Sheet 2 United States Patent 3,164,970 DEFROET CONTROL Earl F. Hubaclker, Evansville, Ind, assignor to Whirlpool Corporation, a corporation of Delaware Filed July 23, 1962, Ser. No. 211,663 2 Claims. (Cl. 612-156) This invention relates to a defrost control apparatus for a refrigerant evaporator.

Various means and methods have been proposed for controlling defrosting automatically of a refrigerant evaporator, particularly when used in refrigerators. and freezers. Generally, the defrosting is done either by an electrically heated wire in thermal contactwith the evaporator or by heated refrigerant. In all of these a control is necessary in order to initiate and terminate the defrosting operation. 7

One of the features of the present invention is to provide an improved defrosting control apparatus for a refrigerant evaporator that employs an ordinary thermostat to initiate the defrosting when an undesirable amount of frost has been accumulated and terminate defrosting after this frost has been melted.

Other features and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings. Of the drawmgs:

' FIGURE -1 is a fragmentary vertical section of a two temperature refrigerator including a defrost control apparatus embodying the invention.

FIGURE 2 is an enlarged detail of a portion of the apparatusof FIGURE 1. 7

FIGURE 3 is a schematic view of the refrigeration system of the refrigerator of FIGURE 1.

FIGURE 4 is a wiring diagram of a portion of the refrigerator circuit.

FIGURE 5 is a graph illustrating'typical temperature refrigeration system of FIGURE 3.

In providing automatic defrosting of'refrigerators and I other devices using refrigerant evaporators, many means have been proposed for initiating and terminating the defrost cycle. A common method has been to 'use a clock timer which automatically defrosts after a predetermined number of hours. This is not satisfactory as conditions vary, and the timer often initiates defrosting when there is no appreciable frost present and other times does not initiate the defrosting soon enough. In some instances, the timer has been arranged so that it only operates when the compressor operates so asto measure the amount of compressor running time. This'is also unsatisfactory for the same reasons as frost conditions often vary.

It has been proposed to provide mechanical devices to sense the degree of frost build-up on the evaporator and initiate defrosting when the frost has reached a predetermined amount and terminate defrosting when the frost has been removed. These generally have been unreliable and not acceptable. Many other defrosting systems have been proposed, but all have been unsatisfactory so that at the present time most manufacturers of refrigerators and freezers prefer to use the expensive and not entirely satisfactory clock timer to control the defrosting.

The defrostcontrol apparatus of this invention provides a simple, reliable and inexpensive arrangement for initiating defrost only when an appreciable amount of frost has accumulated and'terminating defrosting only when this frost has been removed. In the apparatus of conditions as varied by time at points A, B and C in the V Bibifilh Patented Jan. 12, 1965 provided with a bottom 13 with a liner 14 being spaced.

downwardly therefrom to provide duct means here shown as air passage 15 in which is located a refrigerant evapo- V rator 16 includingthe usual refrigerant passage member or evaporator tube 17 and a plurality of secondary heattransfer surfaces in the form of spaced fins 18. Air is.

circulated through both compartments 11 and 12 by means of an air flow means here shown as a fan 19 and motor 20 located in an air passage 21 communicating with the rear'end of the evaporator air passage 15 and emptying through one outlet 22 into the top of the freezer compartment 11 and through a second outlet 23 by way of a rear passage 24 and outlet 25 into the lower refrigerator compartment 12. I

Air from the freezer compartment 11 is drawn back into the evaporator air passage 15 adjacent the front thereof through an opening 26. Air from the refrigerator compartment 12 is drawn back into the passage 15 by way of .a pair of openings 28 and 29 and an air passage 30 the'rebetween. With this arrangement air is continuously drawn from the compartments 11 and 12 and mixed to form a stream that is drawn over the evaporator 16 between the fins 18 and then divided into separate streams for passing through the two compartments.

In order to provide for defrosting of the evaporator 16, there is provided a defrost means here shown as a: resistance heater wire 31 arranged on the evaporator 16 so as to heat the evaporator during 7 the defrost cycle and remove accumulated frost.

The refrigeration system is illustrated in FIGURE 3.

temperature responsive means or switch 37 located in refrigerator compartment 12. Thus, when temperature responsivev switch 37 senses a low refrigerator cabinet air temperature corresponding to the switch cut-outtemperature, the operation'of the compressor is terminated. Likewise, when cabinet air temperature rises to a switch cut-in level, switch 37 closes and initiates compressor operation.

.In order to control the operation of the defrost heater 31, there is provided a second temperature responsive means here shown as a bimetal defrost thermostat 38 provided with two contacts one of which 39 is a cooling contact and the other of which 40 is a heating or defrost contact. A wiring diagram is shown in FIGURE 4. The

this invention this control apparatus is controlled solely 1 bimetal switch contact 41 itself moves between the contacts 39 and 40 as a result of temperature changes. Thus,

contacts 41 and 39 are normally closed to call for compresso'r operation. When the temperature of the-refrigcenter to close contacts 41 and 4t) and open contacts 39 and 41 to stop operation of compressor'32. Closing of;

contacts 4a and 41 energizes heater 31 and defrost com mences. Thereafter when the temperature of the refrigerant tube 17 reaches a preselected above freezing temperature indicative of a frost free condition, the bimetal contact 41 snaps over center and returns to its normal condition, that is, the closing of contacts 39 and 41 stops the defrost and closes the circuit to the compressor.

As is shown most clearly in FIGURE 2, the defrost thermostat 38 is placed so that the sensor 42 thereof is in direct contact with the evaporator tube 17. The defrost thermostat 38 is supported in this contact by means of a bracket 43 suspended from adjacent passes of tube 17 with the bracket also serving to isolate the thermostat from direct contact with the flowing air stream indicated by the arrows 44.

With this arrangement the defrost thermostat 38 initiates defrosting when frost build-up on the tube 17 has become so great as to impair the heat transfer between the surface of tube 17 and the passing air 44. This impaired heat transfer is caused not only by the insulating effect of the frost on the tube but also by the decrease in air flow through the evaporator due to the restriction imposed by the frost. When frost begins to form on the evaporator, the temperature of the surface of tube 1'7 starts to decrease from its normal level because of this decrease in heat transfer. It has been observed that there is a definite relationship between the amount of frost accumulated on the evaporator surfaces and the tem-- perature of the evaporator tube surface.

In normal operation, the compressor 32 and the'air circulating fan 19 are cycled on and off when temperature responsive switch 37 senses cut-in or cut-out temperatures, respectively. At the instant when the temperature responsive switch 37 senses cut-out temperature, the term perature of the surface of tube 17 of the evaporator is a preselected value. If no appreciable frost builds up on the evaporator during the next succeeding cycle, the temperature of the surface of the evaporator tube at compressor cut-out will again pull down to this preselected value. In subsequent cycles frost begins to form on the evaporator tube 17 and fins 18 and, as described previously, this frost build-up impairs heat transfer from the evaporator to the passing air 44. Thus, under such conditions, the surface temperature of the evaporator tube 17 at the time of compressor 'cut-out'will be lower than its preselected value at the time it was operating without frost. Further cycling of the system increases the frost accumulation which decreases heat transfer and in turn further reduces the surface temperature of tube 17 at the time of compressor cut-out. When this tube surface temperature falls to a level substantially below its preselected value at the frost-free condition, the defrost thermostat 3S terminates compressor operation and energizes heater 31 to commence a defrost cycle as described above. Thereafter, when the surface temperature of tube 17 reaches a suitable above-freezing level, the defrost cycle is terminated, alsoas previously explained.

In one embodiment of this invention the preselected temperature of the surface of tube 17 at compressor cutout when the evaporator was in a frost-free condition was 16 F. Inthis same embodiment it was found that when'this tube surface temperature dropped to 26 F., frost had accumulated to such an extent that temperatures within compartments 11 and 12 began to be adversely affected. Therefore, this temperature was used to initiate defrosting.

FIGURE 3 illustrates three points or locations A, B and C in the refrigeration system. Point A is at the refrigerant entrance to the evaporator 16 while point B is adjacent the refrigerant exit from the evaporator and point C is at the accumulator. The refrigerant flow in this system is shown by the arrows 45. Point B is also adjacent the air entrance to the evaporator as indicated by the arrows 44.

When evaporator tube temperature versus the time, as indicated in FIGURE 5, is measured at either points A or C, it is found that immediately after compressor cut-in the refrigerant tube temperature drops rapidly to an extreme low before rising to a higher level 47 (FIGURE 5) where it resumes a more gradual drop. When refrigerant tube temperature is measured at point B, however, the temperature after compressor cut-in begins to fall gradually, as shown in FIGURE 5. At point 47 on the curve, refrigerant tube temperatures at points A, B and C are substantially the same.

Because of the existence of the above-described condition, it is preferred that the defrost thermostat 38 be located adjacent point B in the system. If the thermostat 38 were placed adjacent either points A or C in the system (at the entrance to the evaporator or on the accumulator), there is the possibility that the temperature at these points soon after compressor cut-in might drop below the defrost initiating temperature due to the sharp drop shown on curve A-C of FIGURE 5 and cause the thermostat 38 to initiate a false defrost cycle. In order to prevent these false defrost cycles, the defrost thermostat 38 should be placed at a point on the evaporator where the temperature of the refrigerant tube 17 drops along on even slope, such as at point B shown in FIGURE 5.

It is necessary that good contact be obtained between the sensor 42 and the tube 17 in order that the thermostat will be accurately responsive to the surface temperature of the tube 17. This is particularly true where the thermostat is located, as shown, adjacent the point B because the entering air at this point is at relatively high temperature compared to the refrigerant tube temperature, and it is necessary that the thermostat be relatively unaffected by the air temperature. It is also necessary that substantially no space he left for the accumulation of frost between the sensor 42 and the tube 17.

Thus, in the preferred construction, the defrost thermostat 38 is located adjacent the refrigerant exit of the evaporator and the sensing portion 42 of the defrost thermostat is shielded or otherwise isolated from the air stream as by the bracket 43 in order that the air will not blow directly over it.

Having described my invention as related to the embodiment shown in the accompanying drawings, it is my intention that the invention be not limited by any of the details of description, unless otherwise specified, but rather be construed broadly within its spirit and scope as set out in the accompanying claims.

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. Refrigeration apparatus, comprising: an evaporator having a refrigerant passage member with a refrigerant entrance and exit in which refrigerant evaporates to cool said refrigerant passage member and thus said evaporator to below freezing temperatures whereby frost tends to collect on the evaporator; means for flowing air over said evaporator; duct means surrounding the evaporator for confining air flowing over said evaporator, the duct means including spaced fins in heat exchange relationship with said passage member; refrigerant liquefying means connected to said evaporator; defrost heater means thermally associated with said evaporator for periodically defrosting said evaporator; temperatume responsive means mounted directly on said refrigerant passage member adjacent said refrigerant exit for detecting the temperature of refrigerant in said passage member adjacent said exit for initiating operation of said defrost means upon said temperature responsive means reaching a preselected low temperature indicative of undesirable frost conditions on said evaporator; and a mounting bracket means shielding said temperature responsive means from flowing air in said duct means whereby said temperature responsive means is responsive substantially only to the temperature of refrigerant in said refrigerant passage member.

2. Refrigeration apparatus, comprising: an evaporator having a refrigerant passage member with a refrigerant entrance and exit in which refrigerant evaporates to cool s,1ea,970

collect on the evaporator; means for flowing air over said 1 evaporator; duct means surrounding the evaporator for confining air flowing over said evaporator; refrigerant liquefying means connected to said evaporator; defrost heater means thermally associated with said evaporator for periodically, defrosting said evaporator; temperature responsive means mounted directly on said refrigerant passage member adjacent said refrigerant exit for detecting the temperature of refrigerant in'said passage member adjacent said exit for initiating operation of said defrost means upon saidtemperature responsive means reaching a preselected low temperature indicative of undesirable frost conditions on said evaporator; and means shielding said temperature responsive means from flowing air in said duct whereby said temperature responsive means i a is responsive substantially only to the temperature of refrigerant in said refrigerant passage member.

Reterences Cited in the tile of this patent UNITED STATES PATENTS 2,982,115 Wurtz et a1 May 2, 2,988,897 McGratn June 20, 1961 2,991,630 Wurtz July 11, 1961 3,004,400 Mann et al Oct. 17, 1961 3,023,589 Jacobs Mar. 6, 1962 3,029,611 Kuhn Apr. 17, 1962 3,050,955 Solley et a1. Aug. 28, 1962 Mann et a1; Aug; 28, 1962 

1. REFRIGERATION APPARATUS, COMPRISING: AN EVAPORATOR HAIVNG A REFRIGERANT PASSAGE MEMBER WITH A REFRIGERANT ENTRANCE AND EXIT IN WHICH REFRIGERANT EVAPORATES TO COOL SAID REFRIGERANT PASSAGE MEMBER AND THUS SAID EVAPORATOR TO BELOW FREEZING TEMPEATURES WHEREBY FROST TENDS TO COLLECT ON THE EVAPORATOR; MEANS FOR FLOWING AIR OVER SAID EVAPORATOR; DUCT MEANS SURROUNDING THE EVAPORATOR FOR CONFINING AIR FLOWING OVER SAID EVAPORATOR, THE DUCT MEANS INCLUDING SPACED FINS IN HEAT EXCHANGE RELATIONSHIP WITH SAID PASSAGE MEMBER; REFRIGERANT LIQUEFYING MEANS CONNECTED TO SAID EVAPORATOR; DEFROST HEATER MEANS THERMALLY ASSOCIATED WITH SAID EVAPORATOR FOR PERIODICALLY DEFROSTING SAID EVAPORATOR; TEMPERATURE RESPONSIVE MEANS MOUNTED DIRECTLY ON SAID REFRIGERANT PASSAGE MEMBER ADJACENT SAID REFRIGERANT EXIT FOR DETECTING THE TEMPERATURE OF REFRIGERANT IN SAID PASSAGE MEMBER ADJACENT SAID EXIT FOR INITIATING OPERATION OF SAID DEFROST MEAN UPON SAID TEMPERATURE RESPONSIVE MEANS REACHING A PRESELECTED LOW TEMPERATURE INDICATIVE OF UNDERSIRABLE FROST CONDITIONS ON SAID EVAPORATOR; AND A MOUNTING BRACKET MEANS SHIELDING SAID TEMPERATURE RESPONSIVE MEANS FROM FLOWING AIR IN SAID DUCT MEANS WHEREBY SAID TEMPERATURE RESPONSIVE MEANS IS RESPONSIVE SUBSTANTIALLY ONLY TO THE TEMPERATURE OF REFRIGERANT IN SAID REFRIGERANT PASSAGE MEMBER. 